porting and borrowing more

My mortgage provider has said I can port my mortgage and has said that based on my salary etc, I can borrow an extra £15k more, which is great. If I come out of my mortgage and go elsewhere by paying my ERC - regardless of lower interest rates - id be worse off. Plus I can't afford to take the hit of paying the ERC as I need that money for the purchase.

The man I spoke to at Natwest was quite confusing. When you borrow more on an existing mortgage, is it like you are taking another mortgage out? As in would that be a two year mortgage? At really high rates? The man said he couldn't answer that until I was in a position to proceed with a new purchase - but I need to know the answer so I can work out amount I have....